Spatially Resolved Spectroscopy of a Balmer-Dominated Shock in the Cygnus Loop: An Extremely Thin Cosmic-Ray Precursor?

Abstract

We present high-resolution long-slit spectroscopy of a Balmer-dominated shock in the northeastern limb of the Cygnus Loop with the Subaru high dispersion spectrograph. By setting the slit angle along the shock normal, we investigate variations of the flux and profile of the Hα line from preshock to postshock regions with a spatial resolution of ∼4×10 15 cm. The Hα line profile can be represented by a narrow (28.9±0.7 km s −1 ) Gaussian in a diffuse region ahead of the shock, i.e., a photoionization precursor, and narrow (33.1±0.2 km s −1 ) plus broad (130-230 km s −1 ) Gaussians at the shock itself. We find that the width of the if we eliminate projected emission originating from the photoionization precursor, in an unresolved thin layer ( 4 × 10 15 cm at a distance of 540 pc) at the shock. We show that the sudden broadening can be best explained by heating via damping of Alfvén waves in a thin cosmic-ray precursor, although other possibilities are not fully ruled out. The thickness of the cosmic-ray precursor in the Cygnus Loop (a soft gamma-ray emitter) is an order of magnitude thinner than that in Tycho's Knot g (a hard gamma-ray emitter), which may be caused by different energy distribution of accelerated particles between the two sources. In this context, systematic studies might reveal a positive correlation between the thickness of the cosmic-ray precursor and the hardness of the cosmic-ray energy distribution.

CONSTRAINING THE AGE AND DISTANCE OF THE GALACTIC SUPERNOVA REMNANT G156.2+5.7 BY Hα EXPANSION MEASUREMENTS

“…2), seemingly composed of two long pairs of filaments in the first-epoch images can now be seen to consist of many delicate and overlapping filaments along with faint but noticeable diffuse emission in between filaments. This complex morphology is similar to that seen in the nonradiative filaments along the northern and eastern limbs of the Cygnus Loop SNR where the shock front is seen edge on and gently warped as it moves through an interstellar medium with small density variations on large scales (Hester et al 1986(Hester et al , 1994Levenson et al 1998;Blair et al 1999Blair et al , 2005Medina et al 2014;Katsuda et al 2016).…”

Inverse Compton emission from a cosmic-ray precursor in RX J1713.7−3946

“…Furthermore, linear analysis and several numerical simulations show that the CR precursor generates magnetic-field fluctuation (Bell, 1978(Bell, , 2004Niemiec et al, 2008;Ohira et al, 2009;Riquelme & Spitkovsky, 2009;Ohira & Takahara, 2010;Caprioli & Spitkovsky, 2013). Recently, Katsuda et al (2016) found that the upstream plasma of the SNR Cygnus loop is abruptly heated in the vicinity of shocks explained via damping of magnetic-field fluctuation in an unresolved thin CR precursor. However, the CR precursor of SNRs has never been directly imaged so far.…”

Injection to Rapid Diffusive Shock Acceleration at Perpendicular Shocks in Partially Ionized Plasmas

“…However, it is well known that the ISM and ejecta of SNRs are not always fully ionized. In fact, the observed line profile of H α emission from SNRs supports that some SNR shocks are propagating to the partially ionized ISM (Chevalier & Raymond 1978;Katsuda et al 2016). It was proposed that many plasma instabilities are excited by the ionization around a collisionless shock in the partially ionized plasma Ohira et al 2009;Ohira & Takahara 2010;Ohira 2014).…”